1. Technical Field
The present invention relates to variable optical attenuators and, more particularly, to variable optical attenuators based upon polarization rotation having large dynamic range, compact size and fast tuning speed.
2. Description of Related Art
In many applications, including in fiber optics communication network systems, it is often required to adjust the intensity (optical power) of light signals. For example, one factor determining the quality of a signal is the ratio of the intensity of an optical signal to the intensity of the noise in the optical signal, typically referred to as the optical signal-to-noise ratio (optical SNR). Therefore, it is often necessary to adjust the intensity of a light signal to increase the optical SNR such that the optical SNR exceeds a predetermined level. Moreover, the gain of an optical amplifier typically depends on the wavelength of the signal undergoing amplification resulting in the various component wavelengths of an optical signal having different intensities.
A variable optical attenuator (VOA) is typically used to control (that is, reduce) the intensity of selected light signals and thereby to maintain each light signal at substantially the same light intensity. Several types of optical attenuators are known in the art.
One conventional type of optical attenuator is a mechanical type optical attenuator in which attenuation is achieved by mechanically shifting or rotating the position of an incoming light signal. However, due to the relatively slow speed of mechanical motion, these mechanical types of VOAs have limited tuning speed, generally slower than about 1 millisecond (1 ms). Long term reliability of these types of devices is still uncertain.
Rotation of the plane of polarization of an incoming light beam can also be used to fabricate a VOA having the advantage of achieving attenuation without the need for mechanical motion of any component. For example, Japanese laid-open patent application No. 6-51255 entitled xe2x80x9cOptical Attenuatorxe2x80x9d discloses a VOA that includes a magneto-optical crystal, a polarizer, a permanent magnet and an electromagnet. The incoming light signal is linearly polarized by a polarizer. This linearly polarized light is then passed through a magneto-optical crystal. A permanent magnet applies a constant magnetic field to the magneto-optical crystal in a direction parallel to the light path. An electromagnet applies a variable magnetic field in a direction perpendicular to the light path, the strength of this variable magnetic field readily controlled by controlling the current passing through the electromagnet. The composite magnetic field resulting from the vector sum of the constant and variable magnetic fields rotates the plane of polarization of the linearly polarized light as it passes through the magneto-optical crystal. The amount of rotation is controllable by controlling the current through the electromagnet. In other words, a magneto-optical crystal, a permanent magnet and an electromagnet in combination are used to form a Faraday rotator. We note that large optical scattering losses may occur when the magneto-optical crystal has a large number of optical domains. However, when the magnetic field provided by the permanent magnet is larger than the saturation level for the crystal, the composite magnetic field is always greater than the saturation magnetic field (since the variable magnetic field is perpendicular to the permanent field the composite intensity is never smaller than either component field). In this case of magnetic fields greater than saturation, magnetic domains inside the magneto-optical crystal are substantially integrated into a single large domain, which results in substantially reduced optical scattering losses.
The amount by which the plane of polarization is rotated upon passage through a magneto-optical crystal is in accordance with the physical principle known as the xe2x80x9cFaraday effect,xe2x80x9d given by Eq. 1.
xcfx86=Vxc2x7Lxc2x7H∥xe2x80x83xe2x80x83Eq. 1
where
xcfx86=the rotation angle of the plane of polarization.
L=the path length through the magneto-optical material.
H∥=the magnitude of the component of the magnetic field applied to the magneto-optical crystal in the direction of light propagation.
V=Verdet constant; a constant of proportionality dependent on the particular magneto-optical material.
The VOA described above requires linearly polarized light. Light of arbitrary polarization can be attenuated by the technique depicted in FIG. 1 as described in the above-referenced Japanese Laid-Open patent application. Essentially, a birefringent crystal is used to separate randomly polarized incident light into two orthogonal plane polarizations corresponding to the ordinary and extraordinary optical axes of the birefringent crystal. The birefringent crystal physically separates the ordinary and extraordinary beams which then have polarizations rotated by passage through a Faraday rotator. The two rotated light beams emerging from the Faraday rotator are directed into a second birefringent crystal oriented so each of the two incident beams is separated into ordinary and extraordinary beams. A focusing lens following the second birefringent crystal focuses two of the four emerging beams into the output fiber, while the other two beams are lost, resulting in attenuation. The relative orientations of the two birefringent crystals and the rotation caused by the Faraday rotator determines the degree of attenuation.
A disadvantage of the prior art devices is that the dynamic range of attenuation is limited by the extinction ratio of the polarization-sensitive elements, including the birefringent crystals and the Faraday rotator. Since the light beam passes through each polarization-sensitive device only once (FIG. 1), the overall dynamic range is similar to the dynamic range of a single stage isolator; that is about 30 dB.
The present invention achieves an increase in the dynamic range of attenuation without increasing the number of stages by making use of an innovative reflective VOA configuration. The reflective VOA of the present invention greatly increases the dynamic range (typically to about 60 dB) and makes the device more compact, reducing the length of the transmission architecture by about a factor of two. The present VOAs also achieve fast tuning speeds.
The present invention relates to variable attenuation of an light beam by use of polarization rotators in which the degree of rotation of the polarization is determined by an externally-applied control signal leading to variable attenuation of the light beam under the control of the external signal. Randomly polarized light arrives at input port to the variable optical attenuator (xe2x80x9cVOAxe2x80x9d). The light encounters a birefringent crystal and is split into orthogonal polarization components. Passage of the light through the birefringent crystal causes separation of the extraordinary beam from the ordinary beam by a walk-off distance L upon emerging from the birefringent crystal.
The light beams emerging from the birefringent crystal then impinge on the surface of wave plates (polarization rotators) that are configured to cause a rotation of the plane of polarization by +45xc2x0 and xe2x88x9245xc2x0 rotation. Both light beams then impact a variable polarization rotator in which the plane of polarization is rotated through a angle that can be varied in response to an externally-applied control signal. The maximum operating range (0 to 100% attenuation) is achieved when the variable rotator is capable of rotations in the range xc2x145xc2x0. However, a lesser range of attenuation is achievable with a lesser range of polarization rotation making use of the VOA of the present invention.
Following passage through the variable polarization rotator, the light beams then encounter a second birefringent crystal that has its crystal axis within the horizontal (x, y) plane used as a beam displacer. The beams next encounter two wave plates configured to produce rotation angles of 0xc2x0 and 90xc2x0 respectively. The beams next strike a retroreflector that interchanges the vertical positions of the beams incident thereon. The returning light beams next encounter wave plates in the reverse direction followed by a reverse traverse through the second birefringent crystal that causes a rotation in the same sense and magnitude as on the forward traverse. The beams next encounter the variable polarization rotator, wave plates and first birefringent crystal in the reverse direction.
Attenuation between 0 and 100% is achieved by causing the variable polarization rotator to rotate the plane of polarization between xe2x88x9245xc2x0 and +45xc2x0, typically by altering the magnetic field applied to the rotator.